Metallic nanoparticles are particles having a diameter in the submicron size range, and are either crystalline or amorphous materials. They are composed of pure metal, such as silver, gold, copper etc., or mixture of metals such as alloys, or core of copper covered by a shell of gold or silver.
Currently, nanoscale metal particles draw intense scientific and practical interest due to their unique properties, which differ from those of bulk and atomic species. The unique properties of metal nanoparticles result from their distinct electronic structure and from their the extremely large surface area and high percentage of surface atoms. Metal nanoparticles are characterized by enhanced reactivity of the surface atoms, high electric conductivity, and unique optical properties. Virtually, nanosized materials are well-known materials with novel properties and promising applications in electrochemistry, microelectronics, optical, electronic and magnetic devices and sensors as well as in new types of active and selective catalysts. Creation of stable concentrated nanocolloids of metals with low resistivity may offer new prospects in computer-defined direct-write noncontact technologies, such as ink-jet printing, for deposition of metallic structures on various substrates. Microfabrication of such structures by lithographic technique is a time-consuming and expensive process. Techniques based on expelling small droplets of molten metals onto substrate have met several problems, such as difficulty of adhering droplets onto a substrate, oxidation of the liquid metal, and the difficulty of fabrication a droplet-expulsion mechanism compatible with high temperatures.
Conventional ink-jet inks may contain two types of colored material, dye or pigment, and are characterized by their main liquid, which is the vehicle for the ink. The main liquid may be water (water-based inks), or an organic solvent (solvent-based inks).
The dye or pigment based inks differ with respect to the physical nature of the colored material. Pigment is a colored material that is insoluble in the liquid, while the dye is soluble in the liquid. Each system has drawbacks: pigments tend to aggregate, and therefore clog the nozzles in the orifice plate, or the narrow tubings in the printhead, thus preventing the jetting of the ink while printing. Dyes tend to dry, and form a crust on the orifice plate, thus causing failure in jetting and misdirection of jets.
It is clear that the term “dye” or “pigment” is a general wording for materials, which are soluble or insoluble, respectively, in the solvents comprising the ink. Therefore, metal nanoparticles may be considered, in this context, if introduced into an ink, as pigments of metal, having a size in the nanometer range.
Conventional pigments in ink-jet inks contain particles in the size range of 100-400 nm. In theory, reducing the particle size to 50 nm or less should show improved image quality and improved printhead reliability when compared to inks containing significantly larger particles.
The majority of inks in ink-jet printers are water-based inks. The use of metal nanoparticles as pigments requires the elaboration of ink formulations containing stable concentrated aqueous metal colloid. The synthesis of stable colloidal systems with high metal concentration is a serious problem. A variety of substances have been used to stabilize silver colloids: amphiphilic nonionic polymers and polyelectrolytes, ionic and nonionic surfactants, polyphosphates, nitrilotriacetate, 3-aminopropyltrimethoxysilane, and CS2. Stable water-soluble silver nanoparticles were also obtained by reduction of a silver ions in the presence of amino- and carboxilate-terminated poly(amido amine) dendrimers, and crown ethers. However, the preparations of stable silver colloids, described up to now in the literature, in procedures based on reduction of metal from solution, have low metal concentrations, which amount only to 10−5-10−2 M (about 0.0005-0.1%) even in the presence of stabilizers (it is almost impossible to obtain a stable aqueous silver colloid with the metal concentrations higher then 10−3 M without an additional stabilizer, due to immediate particle aggregation).
Since ink-jet ink compositions contain, in addition to dyes or pigments, other additives, such as humectants, bactericides and fungicides and binders (polymeric additives, which improve the dye or pigment binding to substrate), the stabilizers should be compatible with these substances and should not change noticeably the physicochemical and rheological characteristics of inks (the most important characteristics are viscosity and surface tension).
Several methods of the metallic image generation with the use of ink-jet technology have been described.
One known method is based on an ink containing a reducing agent and receiving material containing the reducible silver compound (AgNO3 or silver di(2-ethylhexyl)-sulphosuccinate), and, on the contrary, an ink and a receiving support containing a silver compound and reducer, respectively. Heating the receiving support during or after the ink deposition resulted in an image formed by silver metal (U.S. Pat. No. 5,501,150 to Leenders, et al; U.S. Pat. No. 5,621,449 to Leenders, et al).
Another approach for the deposition of metal structures is based on ink-jet printing of organometallic precursors dissolved in organic solvent with subsequent conversion of the precursor to metal at elevated temperatures (˜300° C.). To increase the metal (silver) loading of ink and to obtain higher decomposition rates, silver or other metal nanoparticles may be added to the ink along with the organometallic precursor. Near-bulk conductivity of printed silver films has been achieved with such compositions (Vest, R. W.; Tweedell, E. P.; Buchanan, R. C. Int. J. Hybrid Microelectron. 1983, 6, 261; Teng, K. F.; Vest, R. W. IEEE Trans. Indust. Electron. 1988, 35, 407; Teng, K. F.; Vest, R. W. IEEE Electron. Device Lett. 1988, 9, 591, Curtis, C.; Rivkin, T.; Miedaner, A.; Alleman, J.; Perkins, J.; Smith, L.; Ginley, D. Proc. of the NCPV Program Review Meeting. Lakewood, Colo., USA, Oct. 14-17, 2001, p. 249).
Fuller et al. demonstrated ink-jet printing with the use of colloidal inks containing 5-7 nm particles of gold and silver in an organic solvent, α-terpineol, in order to build electrically and mechanically functional metallic structures. When sintered at 300° C., the resistivity of printed silver structures was found to be 3 μΩ·cm, about twice of that for bulk silver (Fuller, S. B.; Wilhelm, E. J.; Jacobson, J. M. J. Microelectromech. Syst. 2002, 11, 54).